[time-nuts] NIST

Brooke Clarke brooke at pacific.net
Thu Aug 30 21:54:17 EDT 2018

Hi Gregory:

I wonder if anyone has tried using a small parabolic dish, like used for Free To Air satellite TV and aimed it at a GPS 
satellite track or at a WAAS geostationary satellite using a feed antenna with reverse polarization from a normal GPS 

Have Fun,

Brooke Clarke
1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------
> On Thu, Aug 30, 2018 at 9:43 PM Brooke Clarke <brooke at pacific.net> wrote:
>> I would disagree in that ease of jamming/spoofing is strongly related to wavelength.  That's because antenna efficiency
>> goes down as the size of the antenna gets smaller than 1/4 wave.
>> So, it's easy to make a GPS jammer (1,100 to 1,600MHz) since a 1/4 wavelength is a few inches, something that  you can
>> hold in your hand.
> However, the short wavelengths of GPS make beam forming a reasonable
> countermeasure against jamming.
> By having a small array of GPS antennas a receiver can digitally form
> beams that both aim directly at the relevant satellites (so even
> reducing intersatellite interference) while also steering a deep null
> in the direction of the jammer.  If the jammer is powerful enough to
> overload the front-end then this won't help, but against a
> non-targeted area denying jammer it should be fairly effective.
> There are many papers on GNSS beamforming. ( e.g.
> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134596/
> https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5134483/ )
> This kind of anti-jamming solution should even be pretty inexpensive
> -- really no more than the cost of N receivers. Except that it is
> specialized technology and thus very expensive. :)
> Seeing some open source software implementing beam-forming was one of
> the things I hoped to see result from the open hardware multi-band
> GNSS receivers like the GNSS firehose project (
> http://pmonta.com/blog/2017/05/05/gnss-firehose-update/ ) since once
> you're going through the trouble of running three coherent receivers
> for three bands, stacking three more of them and locking them to the
> same clock doesn't seem like a big engineering challenge... and the
> rest is just DSP work.
> Even absent fancy beam forming, for GNSS timing with a surveyed
> position except at high latitudes it should be possible to use a
> relatively high gain antenna pointed straight up and by doing so blind
> yourself to terrestrial jammers at a cost of fewer SVs being
> available. But I've never tried it.
> In an urban area I noticed my own GPSDOs losing signal multiple times
> per week. Monitoring with an SDR showed what appeared to be jammers.
> As others have noted intermittent jamming is pretty benign to a GPSDO.
> Spoofing, OTOH, can trivially mess up the timing.  It's my view that
> if you need timing for a security critical purpose there isn't really
> any GNSS based solution commercially available to the general public
> right now, the best bet is a local atomic reference with a GPSDO used
> to monitor and initially set it.
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